big bore RFVC owners..... i need help!

i have this 1989 honda GB400TT cafe street bike and it has an RFVC 400 engine. currently my bike is not running due to the lack of compression and possible damage to its piston. i did some research and so far i only got info from www.oldrice.com that the GB500TT(which was sold there in the U.S during the early 90s) ha a downsized 1989 XR600 engine. and generally, the GB400 and 500 look exactly the same and share a lot of similarities.

now is it possible to put an XR600 piston into my 400 and not worry about anything else?? those who have owned or still own a GB500 before please share your ideas.

take a look at my "garage" if your wondering how a GB looks like. i labeled it as an XR400 though.

is it possible to put an XR600 piston into my 400 and not worry about anything else??

Is the 600 motor just a stroked 400?!?!?!?! That sounds pretty weird. I think guys are boring the 400 motor out to 440...that may help you if your piston/ bore is fried.

Did you throw this same thread up in the xr400 forum too? that may yield some results. good luck.

Unfortunately, I don't think that it is so simple. The XR600 uses a 24mm piston pin. I think that the GB500 does, but not the 400. Not 100% sure of that though.

If I remember the connecting rod from a XR 600 can be used and this allows the use of the XR 500,or 600 pistons.

Do some internet searches on GB400 piston.

I'm not sure, but I bet this guy knows...

Crmc33. Send him a PM.

I’m not particularly knowledgeable on this engine, but (so far as I know)

1. the engine of the GB400 does not belong to the same “family” as the XR/XL/NX 500/600/650 engines. The GB500 engine does.

2. the XR400 has a different engine altogether

Frankly, I should think it would be best to transplant an engine from the XR/XL/NX series.

Check out some info (mostly GB500) on:

http://www.thumperpage.com/articles/gb-500faq.html

I hope this helps

M.

1. the engine of the GB400 does not belong to the same “family” as the XR/XL/NX 500/600/650 engines. The GB500 engine does.

2. the XR400 has a different engine altogether

http://www.thumperpage.com/articles/gb-500faq.html

.

Right. The GB400 engine shares very few parts with the GB500 engine which is a sleeved down XR600L engine.

wow! thanks for the links guys. i really appreciate your help! :smirk:

No helpful info, but that is a kick:censored: bike. :smirk:

once again thanks for the help on the links guys! :smirk:

i finally had the chance to bring the GB to a mechanic and just to find out that one of the exhaust valves was out of spec which explains the "loose compression" feel when i try to crank it up! so he set the valves, cleaned the spark plug and drained the carb then VROOMMM!!! one swift kick with the choke on and my old GB fired up!! wohhooo!!

:smirk: Nice! Nothing like dodging a bullet! Glad to hear you didn't need an engine rebuild.

Just so there's some correct info here, the RFVC GB400 engine is from the same family as the GB500 XR650L engine etc etc, but as mentioned does have a different stroke. The spigot on the crank case is different too- smaller but could be machined out if you wanted a really under square engine. From looking at a 400 and a 650 engine (which I have in pieces at the moment) you could swap the barrel (and head as the combustion chamber is much bigger), with a lot of stuffing around, but it would be much easier to swap the whole engine. To do this, you need to combine the gearbox internals and primary drive from the 400 with the 650 cases, clutch and engine upper. This way you get the correct 1 down 4 up shift pattern, and the final drive ratio comes out right. You want the CDI from the 650 as it has a different advance curve, but you need to do some fiddling with the ignition and generator coil from the 400 to get it to fit .

Trev

Just so there's some correct info here, the RFVC GB400 engine is from the same family as the GB500 XR650L engine etc etc, but as mentioned does have a different stroke. The spigot on the crank case is different too- smaller but could be machined out if you wanted a really under square engine. From looking at a 400 and a 650 engine (which I have in pieces at the moment) you could swap the barrel (and head as the combustion chamber is much bigger), with a lot of stuffing around, but it would be much easier to swap the whole engine. To do this, you need to combine the gearbox internals and primary drive from the 400 with the 650 cases, clutch and engine upper. This way you get the correct 1 down 4 up shift pattern, and the final drive ratio comes out right. You want the CDI from the 650 as it has a different advance curve, but you need to do some fiddling with the ignition and generator coil from the 400 to get it to fit .

Trev

i was about to revive this thread but good thing you posted and gave me some tips that i could really use as of this time. :banghead:

im planning to do a complete engine swap since the engine is kinda screwed up. it keeps eating oil(about half a liter every 2 weeks if i ride it to work) and 4th gear wont engage at high speeds. should i get a new piston and a new transmission or should i just a get another engine? i have a tight budget so i prefer doing something that is cheaper.

i was about to revive this thread but good thing you posted and gave me some tips that i could really use as of this time. :banghead:

im planning to do a complete engine swap since the engine is kinda screwed up. it keeps eating oil(about half a liter every 2 weeks if i ride it to work) and 4th gear wont engage at high speeds. should i get a new piston and a new transmission or should i just a get another engine? i have a tight budget so i prefer doing something that is cheaper.

Well if you are gonna take the engine out anyway i`d strip her apart and see what it needs,it may just be a bent/worn shift fork...you won`t need a new complete tranny for sure!!!:cheers:

As for it burning oil does it smoke at start-up for a few minutes???if it does then most of your oil burning is just valve seals and/or guides which is rather cheap...

If it doesn`t smoke at start-up then you may get away with just rings if the bore is within specs...

You could possibly be opening yourself for dissappointment with another used engine,who knows how long it`ll be problem free if it isn`t a problem right at start-up,it could burn oil worse than the one you have..at least with the engine you have now you know exactly what you have,no surprises...

Good luck..

Brian

Unfortunately, I don't think that it is so simple. The XR600 uses a 24mm piston pin. I think that the GB500 does, but not the 400. Not 100% sure of that though.

If I remember the connecting rod from a XR 600 can be used and this allows the use of the XR 500,or 600 pistons.

Do some internet searches on GB400 piston.

An xr500 pin is smaller (22mm) than the 600 (24mm). God knows what size the 400 has. Looked at the Garage. Love the twin header pipes.

If I was you, before I started spending a lot of money on that bike I'd start enquiring about the import duties etc on bikes into the Philipines and see if it's worthwhile getting another GB out of Japan. If they bring in lots of used scooters into the Philipines then maybe you could persuade an importer to chuck a GB in with them. They aren't that expensive.

The one you have doesn't have the original forks and clipon handlebars on it it, and if I remember rightly you had some problems with weird shaking with it a while ago. I reckon it was stacked, and probably the forks and frame were bent, so they stuck those forks on it and didn't straighten the frame. So I'm thinking with a stuffed cylinder bent frame etc, it might be better used for parts.

If you do want to keep it, there are a few GB400 engines for sale in Australia, low km Jap imports, have a look on the net, they are cheap as no one wants them. Fixing the old one is a problem as you need genuine everything, except for the bottom end gasket set, and finding genuine would be nearly impossible.

Otherwise look around for another RFVC engine, they all fit straight into the frame, you just need to watch out for sprocket alignment and the ignition/ generator system, and maybe get used to a 1 up 4 down shift pattern, unless you do the gearbox coversion and you box is OK.

Trev

Well if you are gonna take the engine out anyway i`d strip her apart and see what it needs,it may just be a bent/worn shift fork...you won`t need a new complete tranny for sure!!!:banghead:

As for it burning oil does it smoke at start-up for a few minutes???if it does then most of your oil burning is just valve seals and/or guides which is rather cheap...

If it doesn`t smoke at start-up then you may get away with just rings if the bore is within specs...

You could possibly be opening yourself for dissappointment with another used engine,who knows how long it`ll be problem free if it isn`t a problem right at start-up,it could burn oil worse than the one you have..at least with the engine you have now you know exactly what you have,no surprises...

Good luck..

Brian

i changed the rings 4 years ago and after a few months it started eating up oil but i have never changed the valve seals though. it smokes whenever i hit the gas especially during hard acceleration.

if i have the time i take off the top end check if everything is ok.

If I was you, before I started spending a lot of money on that bike I'd start enquiring about the import duties etc on bikes into the Philipines and see if it's worthwhile getting another GB out of Japan. If they bring in lots of used scooters into the Philipines then maybe you could persuade an importer to chuck a GB in with them. They aren't that expensive.

The one you have doesn't have the original forks and clipon handlebars on it it, and if I remember rightly you had some problems with weird shaking with it a while ago. I reckon it was stacked, and probably the forks and frame were bent, so they stuck those forks on it and didn't straighten the frame. So I'm thinking with a stuffed cylinder bent frame etc, it might be better used for parts.

If you do want to keep it, there are a few GB400 engines for sale in Australia, low km Jap imports, have a look on the net, they are cheap as no one wants them. Fixing the old one is a problem as you need genuine everything, except for the bottom end gasket set, and finding genuine would be nearly impossible.

Otherwise look around for another RFVC engine, they all fit straight into the frame, you just need to watch out for sprocket alignment and the ignition/ generator system, and maybe get used to a 1 up 4 down shift pattern, unless you do the gearbox coversion and you box is OK.

Trev

well my bike was junk when i got it and it came with no forks and front wheel. the entire front end is from a donor CB350 and the handlebars are from another bike. i do have the original clip ons but i prefer the tall standard style bars for comfort. the front end wobble that it had before was fixed when i got the front wheel trued. :cheers:

i was thinking of getting another GB400 as a donor bike but looking for one here is a pain in the ass and that would be my last resort. if all else fails then getting a 600 engine would be the next best thing. :worthy:

is there a website that you can refer to me for GB400 parts and engines?

http://www.amy.hi-ho.ne.jp/tachi/english/gb400/tune_eng.html#Engine

http://www.kiwibiker.co.nz/forums/showthread.php?t=48228

Looks like the bottom end is almost a 100% copy of the XR600 except for the rod.

The crank is different though. Only a 72mm stroke. The XR500/XL600 are 75mm, the XR600 is 80mm and the 650L is 82mm.

Just for the record, the XR500 has an 80mm stroke. The shortest stroke of all is with the XL600 (75mm), which has the bigger bore of all (100mm, together with the NX/XRL650).

One of the things I’ve been thinking was to get an XL short stroke engine (or just the crank) and fit the superbig-bore, 106mm kit (piston + sleeve) from Kustom Kraft. This is the kit that raises the XRL650 capacity to just over 720cc. The XL capacity would then become 662cc, which isn’t much for these engines, but the engine would become immensely oversquare, with a bore/stroke ratio comparable to a modern V2 superbike. It would rev to the moon (before it explodes!)

I wonder if this could be done with the 72mm crank and the 400cc engine. Then it would become a 636cc rolling hand grenade!

... I need professional help, I know :banghead:...

Manos

Just for the record, the XR500 has an 80mm stroke. The shortest stroke of all is with the XL600 (75mm), which has the bigger bore of all (100mm, together with the NX/XRL650).

One of the things I’ve been thinking was to get an XL short stroke engine (or just the crank) and fit the superbig-bore, 106mm kit (piston + sleeve) from Kustom Kraft. This is the kit that raises the XRL650 capacity to just over 720cc. The XL capacity would then become 662cc, which isn’t much for these engines, but the engine would become immensely oversquare, with a bore/stroke ratio comparable to a modern V2 superbike. It would rev to the moon (before it explodes!)

I wonder if this could be done with the 72mm crank and the 400cc engine. Then it would become a 636cc rolling hand grenade!

... I need professional help, I know :banghead:...

Manos

Just for the record the older pentroof XR500 is 80mm stroke. The RFVC XR500 is 75mm stroke just like the XL600. At least that's for US models. Honda did some weird stuff for non US bikes. For your purpose it doesn't matter since you can't buy off the shelf pistons larger than 95mm for the XR500 as far as I know. The problem with all bigbore kits is top rpm come down the larger displacement gets unless valve area and cam timing are increased proportionately. Just putting a huge piston in is only the start. You need the oversize valves, porting and major cam to keep the revs where they were at the smaller displacement. While they can be improved greatly I'm afraid even a highly modified RFVC head will never be close to a modern superbike head for flow characteristics. Most of the old flatrack XR600s actually used the XL crank and high compression ratios. Displacement wasn't so much the issue as airflow was. Superbikes are using 40-50-60mm strokes to get the high rpm. 75mm is going to keep peak torque/hp around 5500-7000 regardless of head work. You could get overrev to 9 or 10,000 but power would probably be dropping off pretty quickly. I've got a project motor and all the pieces to try it though.

Just for the record the older pentroof XR500 is 80mm stroke. The RFVC XR500 is 75mm stroke just like the XL600. At least that's for US models. Honda did some weird stuff for non US bikes.

I don’t know much about the older design heads. I have only been involved with the RFVC type, which by the way I consider to be very avant guard, even by today’s standards.

My records are mostly based on information from various sources, mainly the internet so I can’t claim for their reliability. I do however own an XR500R workshop manual, which is presently at my mechanic’s hands who is doing some research for my project. When I get my hands on it again I’ll see if there is anything there in the subject to clear this.

For your purpose it doesn't matter since you can't buy off the shelf pistons larger than 95mm for the XR500 as far as I know.

Well, yes but you could fit the cylinder off an XL600 or NX/XRL 650 and get a piston for those. I don’t know if the pin diameters or deck heights match, but surely you could make a custom conrod and/or piston. Agreed, it won’t be off the shelf items, but with this kind of experimentations and modifications you wouldn’t fit stock items anyway.

The problem with all bigbore kits is top rpm come down the larger displacement gets unless valve area and cam timing are increased proportionately. Just putting a huge piston in is only the start. You need the oversize valves, porting and major cam to keep the revs where they were at the smaller displacement. While they can be improved greatly I'm afraid even a highly modified RFVC head will never be close to a modern superbike head for flow characteristics. Most of the old flatrack XR600s actually used the XL crank and high compression ratios. Displacement wasn't so much the issue as airflow was. Superbikes are using 40-50-60mm strokes to get the high rpm. 75mm is going to keep peak torque/hp around 5500-7000 regardless of head work. You could get overrev to 9 or 10,000 but power would probably be dropping off pretty quickly.

Yes again, but for any given engine the smallest stroke is a recipe for rpm and the biggest bore will give you the extra capacity for additional power (well... torque actually). The result will also depend on the weight of the moving parts and, as you say, compression, cam, valves, porting and the lot. Granted, there will be compromises somewhere along the line but I’m pretty sure you can’t get any better for this series of engines.

The bottom line is that going this way should result in a screamer old-school engine, or as close to it as a 25-year old design can get. (Alas, I believe that the hardest part would be keeping the engine together!!!)

I've got a project motor and all the pieces to try it though.

I’d VERY much like to see the results of your project motor!!! When do you plan to start?

Manos

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